1. Field of the Invention
This invention relates generally to wind turbines that operate at variable speed under varying wind conditions, and relates more particularly to a power converter for converting wind energy into AC electrical power at a controlled power factor and for controlling the torque generated by the wind turbine.
2. Description of the Relevant Art
Wind turbines provide a primary source of energy that can be converted into electricity and supplied to utility power grids. Conversion of wind energy to electrical energy is accomplished in a wind turbine by driving an electrical generator, commonly an AC induction generator. If the electrical power generated by a wind turbine is to be supplied to a utility power grid, then it is required to have a constant frequency, e.g., 60 Hertz, that is synchronized to the utility line frequency. This can be accomplished by driving the generator at a constant rotational speed, which, unless a variable speed transmission is used, requires that the wind turbine rotate at a constant speed. Unfortunately, constant speed operation of a wind turbine limits its energy conversion efficiency due to variable wind conditions. Turbine rotor speed needs to be proportional to wind speed for optimal energy recovery.
Variable speed wind turbines have been proposed as a way of increasing the energy conversion efficiencies of constant speed wind turbines. By varying the rotor speed in varying wind conditions, improved energy recovery can be achieved over a range of wind speed. Also importantly, the peak mechanical stresses caused by wind gusts can be reduced by limiting the torque reacted on the wind turbine by the generator and allowing the wind turbine to speed up in response to wind gusts. The increased kinetic energy of the rotor caused by wind gusts serves as a short term energy storage medium to further improve energy conversion. Such operation, however, requires a responsive torque control system.
Although variable speed wind turbines are advantageous from the perspective of increased energy conversion and reduced stresses, the electrical generation system is more complicated than that of a constant speed wind turbine. Since a generator is usually coupled to a variable speed rotor through a fixed-ratio gear transmission, the electrical power produced by the generator will have a variable frequency. This requires a conversion from the variable frequency AC output by the generator to a constant frequency AC for supplying the utility power grid. The conversion can be accomplished either directly by a frequency converter or through an intermediate conversion to DC by a rectifier and reconversion to fixed-frequency AC by an inverter.